The Deep-dose equivalent (DDE) is a measure of external radiation exposure defined by US regulations.
It represents the dose equivalent at a tissue depth of 1 cm (1000 mg/cm2) due to external whole-body exposure to ionizing radiation.
[1] Dose due to external radiation tends to decrease with depth because of the shielding effects of outer tissues.
The reference depth of 1 cm essentially discounts alpha and beta radiation that are easily shielded by the skin, clothing, and bone surface, while taking minimal credit for any self-shielding from the more penetrating gamma rays.
This makes the deep-dose equivalent a conservative measure of internal organ exposure to external radiation, while eye and skin exposure to external radiation must be accounted differently.